4 Answers
4

As has already been said by others, the latter one obviously comes from the imagination that you have to lough so hard you can't hold back your urine anymore. Like in your example you can say

Dieses Video ist zum Wegschmeißen.

Which is a quite common expression and will be understood without mentioning the verb lachen. I think this is when the two expressions above have been mixed up to zum Wegpissen.
According to Google, the expression zum Bepissen is much more common than zum Wegpissen (425,000 vs 15,400) which leads me to the conclusion that it has arisen as assumed above.